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What you are describing is known as a "breast bud." It is the first step of puberty for many young girls and usually starts around age 10, so your daughter seems to be right on time!
Menstruation typically starts about 2 years after breast buds develop.
It is normal to start on one side and/or have one side bigger than the other.
Pain is also normal. I tell patients that it is "growing pains" , as many young girls are afraid their lumps are cancer.
Tylenol or ibuprofen will help if the discomfort is too much..Also, the nipples may be sensitive and applying a thin layer of Vaseline and wearing a protective Tshirt or training bra will help keep clothing from rubbing and irritating them.